Polycrystalline cuprous oxide nanowire array production method using low-temperature electrochemical growth

ABSTRACT

There are provided a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth, and more particularly, to a manufacturing method allowing easy vapor deposition at low temperatures and also a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth which retains characteristics such as large-area growth, high-crystallinity nanowire, uniform radial distribution, easy length, radius adjustment, and the like. 
     A monocrystalline copper oxide (I) nanowire array manufacturing method of the present invention includes a step of manufacturing a nanopore alumina layer (anodized alumina (AAO)) from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method; and a step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer as a nanopore molding flask by means of a low-temperature electrochemical growth method.

FIELD OF THE INVENTION

The present invention relates to a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth, and more particularly, to a manufacturing method allowing easy vapor deposition at low temperatures and also a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth which retains characteristics such as large-area growth, high-crystallinity nanowire, uniform radial distribution, easy length, radius adjustment, and the like.

BACKGROUND OF THE INVENTION

Typically, a nanowire is a wire structure with a diameter of the order of nanometer (nm). Due to its geometrical nanostructure having a high aspect ratio and a large surface area, the nanowire is an important nano material in a wide range of future industrial fields (e.g., a semiconductor memory field, an LED field, a solar cell field, a sensor field, a catalyst field, a battery electrode material field, etc.). As for a nano device having wire structures made of various nano materials as basic constituent units, the constituent units have the same structure and size and a monocrystalline nanowire assuring an electrical property and continuity of electron transport is very important.

In particular, a monocrystalline nanowire array obtained by a self-assembling process has been recognized as a main functional unit of a high-efficiency and high-integration nano device. Among various monocrystalline nanowire array manufacturing methods, an electrochemical growth method using a nanopore membrane (AAO) as a nano molding flask is characterized by low costs and high efficiency and is capable of adjusting a size and a length of a nanowire ranging from nanometer to micrometer in a predetermined pattern.

Until the present, a technology for manufacturing monocrystalline nanowires or nanowire arrays requires high temperature and high pressure conditions or complicated and expensive manufacturing process and equipment.

BRIEF SUMMARY OF THE INVENTION

A monocrystalline copper oxide (I) nanowire array manufacturing method suggested in the present invention is a method for manufacturing a monocrystalline oxide nanowire array having high production yield a radius of which is very uniform and a length of which can be adjusted in the range of from several ten nanometers to several micrometers by inducing complex formation and a decomposition reaction within an electrochemical aqueous solution.

According to an aspect of the present invention, there is provided a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth, in which the method includes: a step of manufacturing a nanopore alumina layer (anodized alumina (AAO)) from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method; and a step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer as a nanopore molding flask by means of a low-temperature electrochemical growth method.

The step of manufacturing a nanopore membrane from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method includes: a step of electrolytically polishing the high-purity aluminum sheet by applying direct current (DC) voltage thereto in an electrolytic polishing solution; a step of primary anodic oxidation for anodically oxidizing the electrolytically polished aluminum sheet in a sulfuric acid (H₂SO₄) aqueous solution or an oxalic acid (H₂C₂O₄) aqueous solution; a step of etching and removing a porous alumina layer formed by the primary anodic oxidation process with a mixed solution of phosphoric acid (H₃PO₄) and chromic acid (CrO₃); a step of secondary anodic oxidation for anodically oxidizing the alumina sheet, from which an alumina oxide layer is removed, in a sulfuric acid (H₂SO₄) aqueous solution or an oxalic acid (H₂C₂O₄) aqueous solution; a step of protecting the nanopore alumina layer from an etching process by coating a mixture of nitrocellulose and polyester thereon after the step of secondary anodic oxidation; a step of forming a nanopore channel by etching the nanopore alumina layer at a predetermined temperature with a phosphoric acid (H₃PO₄) solution; and a step of depositing a platinum (Pt) layer or a gold (Au) layer on one side surface of the nanopore membrane.

The electrolytic polishing solution includes chloric acid (HClO₄) and ethanol at a volume ratio of 1:4.

The step of electrolytically polishing includes electrolytically polishing the high-purity aluminum sheet at a temperature of 10° C. for 4 minutes by applying direct current voltage of +20 V thereto in an electrolytic polishing solution.

The step of primary anodic oxidation includes anodically oxidizing the electrolytically polished aluminum sheet at a temperature of 10° C. for 12 hours by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.

The step of etching and removing a porous alumina layer formed by the primary anodic oxidation process with a mixed solution of phosphoric acid (H₃PO₄) and chromic acid (CrO₃) includes etching and removing a porous alumina layer formed by the primary anodic oxidation process at a predetermined temperature with a mixed solution of phosphoric acid (H₃PO₄) and 1.8 wt % of chromic acid (CrO₃).

The step of secondary anodic oxidation includes anodically oxidizing the aluminum sheet, from which an alumina oxide layer is removed, at a temperature of 10° C. for a desired time period by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.

The step of protecting the nanopore alumina layer from an etching process includes protecting the nanopore alumina layer from an etching process by coating a mixture of nitrocellulose and polyester thereon after the step of secondary anodic oxidation.

The step of forming a nanopore channel includes forming a nanopore channel by etching the nanopore alumina layer with 5 wt % of a phosphoric acid (H₃PO₄) solution at a temperature of 30° C. for 15 minutes.

The step of depositing a Pt layer or an Au layer includes depositing a platinum (Pt) layer or a gold (Au) layer on one side surface of the nanopore membrane to a thickness of 200 nm or more.

The step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer as a nanopore molding flask includes: a step of manufacturing an electrochemical deposition solution by mixing copper nitrate hydrate (Cu(NO₃)₂.2.5H₂O) and hexamethylenetetramine; a step of stirring the electrochemical deposition solution and heating the electrochemical deposition solution in a boiling water bath; a step of stirring the electrochemical deposition solution at a predetermined temperature; a step of applying a predetermined current density to the nanopore molding flask in an electrochemical reaction solution; a step of washing an electrochemically grown nanowire with ethanol and deionized water and drying the nanowire; a step of performing a heat treatment to improve crystallinity of the nanowire; and a step of removing a nanopore membrane with an NaOH aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a low-temperature electrochemical reaction for manufacturing a monocrystalline copper oxide (I) nanowire array;

FIGS. 2 to 4 provide scanning electron micrographs (SEMs) (FIGS. 2 and 3) and an X-ray diffraction diagram (FIG. 4) of monocrystalline copper oxide (I) nanowire arrays manufactured by an electrochemical deposition method using complex formation and a decomposition reaction;

FIGS. 5 and 6 provide transmission electron micrographs (TEMs) of manufactured monocrystalline copper oxide (I) nanowire arrays; and

FIGS. 7 and 8 provide high resolution transmission electron micrographs (HRTEMs) of monocrystalline copper oxide (I) nanowires.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the present invention, a high-integration and high-quality copper oxide nanowire array is manufactured by a low-temperature electrochemical reaction using complex formation and a decomposition reaction.

Typically, a technology for manufacturing monocrystalline nanowires or nanowire arrays requires high temperature and high pressure conditions or complicated and expensive manufacturing process and equipment. Although, however, monocrystalline nanowires manufactured according to the present invention are grown at a low temperature in an aqueous solution composed of a small amount (typically, in the unit of mg) of eco-friendly samples, the monocrystalline nanowires having very high crystallinity are arrayed and grown with uniform size and gap and adjusted length.

According to an example of the present invention, a monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth is roughly divided into a step of manufacturing a nanopore membrane with desired size and thickness and a step of manufacturing a monocrystalline copper oxide (I) nanowire array using a low-temperature electrochemical growth method.

The present invention will be described in detail with reference to the following Examples, but the scope of the present invention is not limited to these Examples.

Example 1

Example 1 was a step of manufacturing a nanopore membrane (anodized alumina (AAO)) from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method.

In other words, after a high-purity aluminum sheet having a desired size was prepared, the high-purity aluminum sheet was electrolytically polished at a temperature of 10° C. for 4 minutes by applying direct current (DC) voltage of +20 V thereto in an electrolytic polishing solution (including chloric acid (HClO₄) and ethanol at a volume ratio of 1:4).

Primary anodic oxidation was performed onto the electrolytically polished aluminum sheet at a temperature of 10° C. for 12 hours by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.

A porous alumina layer formed by the primary anodic oxidation was etched and removed with a mixed solution of 6 wt % of phosphoric acid (H₃PO₄) and 1.8 wt % of chromic acid (CrO₃) at a temperature of 60° C. for 24 hours.

Secondary anodic oxidation was performed onto the aluminum sheet, from which an alumina oxide layer was removed, at a temperature of 10° C. for a desired time period by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.

The nanopore alumina layer formed as described above was protected from an etching process by coating a mixture of nitrocellulose and polyester thereon.

The manufactured nanopore alumina layer was etched with 5 wt % of a phosphoric acid (H₃PO₄) solution at a temperature of 30° C. for 15 minutes or more so as to form a nanopore channel.

A platinum (Pt) layer or a gold (Au) layer was deposited on one side surface of the manufactured nanopore membrane to a thickness of 200 nm or more. These metal layers were used as working electrodes in an electrochemical growth reaction.

Example 2

Example 2 was a step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer obtained from Example 1 as a nanopore molding flask. A 20 mM aqueous solution was prepared by mixing copper nitrate hydrate (Cu(NO₃)₂.2.5H₂O) and hexamethylenetetramine.

Then, the prepared electrochemical deposition solution was heated in a boiling water bath until a temperature thereof reached 80° C. with stirring at a speed of 100 rpm.

Further, when the temperature of the electrochemical deposition solution reached 80° C., the electrochemical deposition solution was stirred at a speed of 100 rpm for 10 minutes.

Thereafter, a predetermined current density of 1 mA/cm² was applied to the prepared nanopore molding flask in an electrochemical reaction solution for a desired time period.

Subsequently, an electrochemically grown nanowire was washed with ethanol and deionized water and then dried.

Then, a heat treatment was performed onto the manufactured nanowire to further improve crystallinity of the nanowire at a temperature of 200° C. for 10 minutes. Herein, a nanopore membrane was removed with a 1.0 M NaOH aqueous solution.

The electrochemical growth method for manufacturing a monocrystalline copper oxide (I) nanowire array as described above is based on the present inventors' patent application (Korean Patent Application No. 10-2009-0022569) relating to a method for forming a high-crystallinity copper oxide (I) thin film.

To be specific, as illustrated in FIG. 1, copper ions (Cu²⁺) and hydroxyl ions (OH—) required to form copper oxide (I) are generated through complex formation and a decomposition reaction within an electrochemical aqueous solution, and two-dimensional nucleation and growth for monocrystalline growth is carried out effectively through an adsorption reaction between the formed complex and a specific growing surface of the copper oxide (I).

The copper ions (Cu²⁺) generated through complex formation and a decomposition reaction are reduced to cuprous oxide ions (Cu⁺) and grown to become a copper oxide (I) structure through a condensation reaction with the hydroxyl ions (OH—) on a conductive metal film.

As can be seen from FIG. 2, high-density nanowires having very uniform radius are arrayed and grown at regular positions in a regular pattern.

Radiuses of the manufactured nanowires can be determined by a pore size of the nanopore membrane and lengths thereof can be adjusted by an electrochemical reaction time.

Radiuses of the monocrystalline copper oxide (I) nanowires manufactured by the above-described method can be adjusted in the range of about 20 nm to about 450 nm and lengths thereof can be readily adjusted in the range of from several ten nanometers to several micrometers. Nanowires as shown in FIG. 3 have a small radius range of about 25±3 nm and are grown to a length of at least 3 μm.

Further, such a nanowire array can be grown to have a large area of the order of centimeter and the area is determined by an area of a nanopore membrane.

FIG. 4 is an X-ray diffraction diagram of manufactured monocrystalline copper oxide (I) nanowire arrays. Incubation and growth directions of the manufactured nanowires are determined by crystallinity of a metal film (a working electrode) deposited on one side surface of a nanopore membrane.

In other words, as can be seen from FIG. 4, the deposited metal film are incubated and grown in directions [111] and [200], and thus, the incubation and growth directions of the manufactured nanowires follow these two crystal growth directions.

As can be seen from FIGS. 5 and 6, the manufactured nanowires are straightly grown in a longitudinal direction and have very smooth surfaces.

Crystallinities of the manufactured nanowires were observed by using a high resolution transmission electron microscope (HRTEM).

High resolution transmission electron micrographs (HRTEMs) (FIGS. 7 and 8) of the manufactured nanowires show that crystal lattices of the manufactured nanowires are uniformly arrayed with gaps of 0.247 nanometers and 0.210 nanometers, respectively.

It can be seen from a distance between crystal faces that the manufactured monocrystalline copper oxide (I) nanowires are incubated and grown in directions and [200]. Herein, the gaps of 0.247 nanometers and 0.210 nanometers between the crystal lattices respectively correspond to a surface (111) and a surface (200) of cubic copper oxide (I).

Therefore, according to the manufacturing method of the present invention, it is possible to manufacture a monocrystalline oxide nanowire array having high production yield, a radius of which is very uniform and a length of which can be adjusted in the range of from several ten nanometers to several micrometers, and also possible to achieve characteristics such as large-area growth, high-crystallinity nanowire, uniform radial distribution, and easy length and radius adjustment.

According to the present invention, a monocrystalline oxide nanowire array having high production yield, a radius of which is very uniform and a length of which can be adjusted in the range of from several ten nanometers to several micrometers, can be manufactured at low temperatures.

According to the present invention, it is possible to achieve characteristics such as large-area growth, high-crystallinity nanowire, uniform radial distribution, and easy length and radius adjustment.

The above description of the present invention is provided for the purpose of illustration, and those skilled in the art can made various changes, modifications, and substitutions without changing essential features of the present invention. Thus, the accompanying drawings are provided not to limit but to explain a technical conception of the present invention, and a range of the technical conception of the present invention is not limited by the accompanying drawings.

The scope of the present invention is defined by the following claims, and it shall be understood that all technical conceptions conceived from the meaning and scope of the claims and their equivalents are included in the scope of the present invention. 

1. A monocrystalline copper oxide (I) nanowire array manufacturing method using low-temperature electrochemical growth, the method comprising: a step of manufacturing a nanopore alumina layer (anodized alumina (AAO)) from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method; and a step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer as a nanopore molding flask by means of a low-temperature electrochemical growth method.
 2. The method of claim 1, wherein the step of manufacturing a nanopore membrane from a high-purity aluminum (Al) sheet by using a two-step anodic oxidation method includes: a step of electrolytically polishing the high-purity aluminum sheet by applying direct current voltage thereto in an electrolytic polishing solution; a step of primary anodic oxidation for anodically oxidizing the electrolytically polished aluminum sheet in a sulfuric acid (H₂SO₄) aqueous solution or an oxalic acid (H₂C₂O₄) aqueous solution; a step of etching and removing a porous alumina layer formed by the primary anodic oxidation with a mixed solution of phosphoric acid (H₃PO₄) and chromic acid (CrO₃); a step of secondary anodic oxidation for anodically oxidizing the alumina sheet, from which an alumina oxide layer is removed, in a sulfuric acid (H₂SO₄) aqueous solution or an oxalic acid (H₂C₂O₄) aqueous solution; a step of protecting the nanopore alumina layer from an etching process by coating a mixture of nitrocellulose and polyester thereon after the step of secondary anodic oxidation; a step of forming a nanopore channel by etching the nanopore alumina layer at a predetermined temperature with a phosphoric acid (H₃PO₄) solution; and a step of depositing a platinum (Pt) layer or a gold (Au) layer on one side surface of the nanopore membrane.
 3. The method of claim 2, wherein the electrolytic polishing solution includes chloric acid (HClO₄) and ethanol at a volume ratio of 1:4.
 4. The method of claim 2, wherein the step of electrolytically polishing includes electrolytically polishing the high-purity aluminum sheet at a temperature of 10° C. for 4 minutes by applying direct current voltage of +20 V thereto in an electrolytic polishing solution.
 5. The method of claim 2, wherein the step of primary anodic oxidation includes anodically oxidizing the electrolytically polished aluminum sheet at a temperature of 10° C. for 12 hours by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.
 6. The method of claim 2, wherein the step of etching and removing a porous alumina layer formed by the primary anodic oxidation with a mixed solution of phosphoric acid (H₃PO₄) and chromic acid (CrO₃) includes etching and removing a porous alumina layer formed by the primary anodic oxidation at a predetermined temperature with a mixed solution of phosphoric acid (H₃PO₄) and 1.8 wt % of chromic acid (CrO₃).
 7. The method of claim 2, wherein the step of secondary anodic oxidation includes anodically oxidizing the aluminum sheet, from which an alumina oxide layer is removed, at a temperature of 10° C. for a desired time period by applying voltage of +20 V thereto in a 0.3 M sulfuric acid (H₂SO₄) aqueous solution or a 0.3 M oxalic acid (H₂C₂O₄) aqueous solution.
 8. The method of claim 2, wherein the step of protecting the nanopore alumina layer from an etching process includes protecting the nanopore alumina layer from an etching process by coating a mixture of nitrocellulose and polyester thereon after the step of secondary anodic oxidation.
 9. The method of claim 2, wherein the step of forming a nanopore channel includes forming a nanopore channel by etching the nanopore alumina layer with 5 wt % of a phosphoric acid (H₃PO₄) solution at a temperature of 30° C. for 15 minutes.
 10. The method of claim 2, wherein the step of depositing a Pt layer or an Au layer includes depositing a platinum (Pt) layer or a gold (Au) layer on one side surface of the nanopore membrane to a thickness of 200 nm or more.
 11. The method of claim 1, wherein the step of manufacturing a monocrystalline copper oxide (I) nanowire array by using the nanopore alumina layer as a nanopore molding flask includes: a step of manufacturing an electrochemical deposition solution by mixing copper nitrate hydrate (Cu(NO₃)₂2.5H₂O) and hexamethylenetetramine; a step of stirring the electrochemical deposition solution and heating the electrochemical deposition solution in a boiling water bath; a step of stirring the electrochemical deposition solution at a predetermined temperature; a step of applying a predetermined current density to the nanopore molding flask in an electrochemical reaction solution; a step of washing an electrochemically grown nanowire with ethanol and deionized water and drying the nanowire; a step of performing a heat treatment to improve crystallinity of the nanowire; and a step of removing a nanopore membrane with an NaOH aqueous solution.
 12. A monocrystalline copper oxide (I) nanowire array manufactured by the manufacturing method according to claim
 1. 